In recent years, a CD or DVD-type optical recording medium capable of writing or reading information using a laser has been widely used. Traders handling a variety of optical recording media in small amounts are required to be able to print characters or images showing the content recorded by an optical recording medium using an inkjet method. As an increasing number of end users started to use an optical recording medium, printers capable of printing information onto such a recording medium have been introduced into the market. In order to optimize the design of the label side of a disk (side opposite to the side on which optical writing/reading is performed), there is an increasing demand for printing not only characters representing the recorded content but also high-quality images like photographs.
With such a background, a number of optical recording media which have a label side capable of being printed with an inkjet printer have also onto the market. Since ink used in the inkjet method is normally aqueous, a water-absorptive ink-receiver layer is formed on the surface of the label side. As the coating composition for forming an ink-receiver layer, a composition which cures upon exposure to radiation such as UV rays is widely used from the viewpoint of productivity.
The ink-receiver layer is required to have properties of quickly drying ink used in the inkjet method, capability of displaying clear images, and properties of preserving printed images such as water resistance. As the coating composition satisfying these requirements, a coating composition which improves resolution while promoting ink absorption by providing irregularities on the surface of a hydrophilic ink-receiver layer is described in JP-A-06-60432.
A coating composition capable of improving quick-drying properties and image clarity obtained by adding an inorganic filler or an organic filler to a hydrophilic resin has been proposed. As an example of the inorganic filler, a coating composition containing a porous smecktite is described in JP-A-07-272441.
A coating composition containing porous alumina as an inorganic filler is described in JP-A-11-213445. A coating composition containing water-absorptive resin particles as an organic filler is described in JP-A-07-169100. A coating composition containing a naturally occurring material as an organic filler is also described in JP-A-10-259340.
In order to improve water resistance of an ink-receiver layer, a number of coating compositions containing organic cations has also been proposed. Normally, an acidic dye is used as ink for the inkjet method. These coating compositions have been proposed to cause dye particles having an opposite charge to fix an ink-receiver layer by adding organic cations to the ink-receiver layer. JP-A-2001-311022 describes specific examples of such a coating composition.